Various heterogeneous multimetallic catalysts have come into commercial acceptance for chemical and petroleum processes previously utilizing monometallic catalysts. For example, see U.S. Pat. No. 3,769,201, Catalytic Reforming Process, J. H. Sinfelt and A. E. Barnett; U.S. Pat. No. 3,729,408, Catalytic Reforming Process, J. H. Sinfelt and J. L. Carter; U.S. Pat. No. 3,617,518, Inhibition of Hydrogenolysis, J. H. Sinfelt and A. E. Barnett; U.S. Pat. No. 3,567,625, Hydroforming with Promoted Iridium Catalysts, J. H. Sinfelt and A. E. Barnett; U.S. Pat. No. 3,442,973, Isomerization Process Utilizing a Gold-Palladium Alloy in the Catalyst, J. H. Sinfelt, A. E. Barnett and G. W. Dembinski; J. H. Sinfelt, J. L. Carter and D. J. C. Yates, J. Catalysis 24, 283 (1972); J. H. Sinfelt, J. Catalysis 29, 308 (1973).
It has been found that in reforming processes, for example, various results may be obtained with multimetallic catalysts similar to those that could not be obtained with the prior art monometallic catalysts. For example, the cracking activity of platinum on alumina could be modified by using a Group I-B metal in combination with platinum as the active catalyst metal. Various catalytic oxidation multimetallic catalysts are now being utilized. For example, see U.S. Ser. No. 259,929, filed June 5, 1972 in the name of James Cusumano, hereby incorporated by reference, which teaches a silver alloy catalyst used for ethylene oxidation. See also U.S. Pat. Nos. 2,605,239; 3,144,416; 3,664,970; 2,424,083; and 2,143,371, teaching other silver alloy catalysts for the same process.
Multimetallic catalysts have also been used in fuel cell processes. For example, as electrodes, PdRu, PtRu, PdAu, PtPd, PdAg and a host of others have been reported.
In many of the above multimetallic catalysts, the composition of the surface of the metal phase does not correspond to that of the bulk metal. Because the bulk composition has only secondary effects on catalysis, it would be useful to have a means of preparing multimetallic catalysts in which one could control the surface composition while maintaining an essentially pure monometallic phase in the bulk. Such a technique would have the following advantages:
Minimize the need for one constituent in catalyst preparation (especially useful if this constituent were rare or expensive).
Permit control of surface composition and thus control of catalytic activity and selectivity.
For example, it is known in the art that one can achieve dramatic enhancement in selectivity of hydrocarbon conversion reactions by alloying Group I-B metals with Group VIII metals (J. H. Sinfelt, J. L. Carter and D. J. C. Yates, J. Catalysis 24, 283 (1972)). These effects are known to correlate with alloy surface composition. Thus, if one alloys copper with nickel (J. H. Sinfelt, J. L. Carter and D. J. C. Yates, J. Catalysis 24, 283 (1972) or copper with ruthenium (J. H. Sinfelt, J. Catalysis 29, 308 (1973)), the copper inhibits the carbon-carbon bond hydrogenolysis activity of the Ru and Ni but maintains and in some instances promotes, hydrogenation and dehydrogenation reactions. This selectivity phenomena is known to be related and dependent upon surface alloy or cluster composition (J. H. Sinfelt, J. L. Carter and D. J. C. Yates, J. Catalysis 24, 283 (1972); J. H. Sinfelt, J. Catalysis 29, 308 (1973)). However, conventionally one adjusts the surface composition by controlling the bulk composition. This is inefficient as one could achieve the same end results with the use of much less metal by controlling surface composition with the invention described herein.